In plants for the recovery of ethylene and other unsaturated hydrocarbons from the cracking of a petroleum feed stock, the petroleum fractions which are used and are subjected to the pyrolysis process can have a higher boiling point than naphtha in many cases.
The use of, for example, gas oil as the feed stock for the pyrolysis process, has already been recognized to result in significant technological problems. In the pyrolysis of naphtha and lighter feed stocks, it is customary to quench the the reaction gas stream (cracking gas) upon its discharge from the cracking furnace by indirect cooling with generation of high pressure steam as rapidly as possible so that the heat recovery will be as high as possible and undesired secondary reactions can be avoided. Reference may be had in this regard to pages 429 ff, of the Chemical and Process Technology Encyclopedia; Considine, Editor, McGraw-Hill Book Company, New York, 1974.
Subsequent cooling is carried out usually by the direct spraying of oil generated in the cracking process into the cracking-gas stream.
It has been found that the quenching of gas oil cracking gases gives rise to a high degree of polymerization and carbon deposition even at high temperatures. In other words, in spite of the fact that a quenching is carried out, the quenching operation remains susceptible to the production of polymers of the unsaturated hydrocarbons contained in the cracking gas and carbonization of the surfaces of the reactor, i.e. cokefication.
Heat abstraction by indirect cooling is limited by the short residence time the cracking gas can be present in the reactor without carbonization or carbon deposition, and is not possible when the starting materials for the cracking operation are very heavy hydrocarbons or like feed stocks.
Thus the greater part of the heat abstraction from the gas stream must be effected by direct spraying of oil into it in accordance with prior art teachings. In the latter case, however, the cooling-oil droplets come into contact with the tube walls in the presence of hot cracking gases and carbonization of these walls can occur. German open application DT-OS No. 2,062,937, for example, describes a technique whereby the cooling oil is so introduced into the cracking gas duct that a film of cooling oil is formed on the tube wall and hence a three-phase interface is not permitted to develop between the cracking gas, the spraying liquid, and the tube wall. This has the tendency to reduce the degree of carbonization of the wall. However, it is not always possible to maintain the continuity of such a cooling oil film so that there are occasions at which the three-phase interface will develop and coke or carbon accumulations on the wall can be found when the cooling oil contacts the wall in the presence of the hot cracking gases.